Support getting current vsync status in app

libs/app/include/psemek/app/app.hpp

@@ -41,6 +41,7 @@ namespace psemek::app
 		std::unique_ptr<scene> pop_scene() override;
 
 		void show_cursor(bool show);
+		bool vsync() const;
 		void vsync(bool on);
 
 		float time() const;

libs/app/source/app.cpp

@@ -274,6 +274,11 @@ namespace psemek::app
 		return std::chrono::duration_cast<std::chrono::duration<float>>(clock::now() - impl().start_time).count();
 	}
 
+	bool app::vsync() const
+	{
+		return SDL_GL_GetSwapInterval() != 0;
+	}
+
 	void app::vsync(bool on)
 	{
 		if (on)